1. Field of the Invention
The present invention relates to a ring type ATM network system where an ATM cell transmission mode is employed.
2. Description of the Prior Art
FIG. 35 is a diagram showing a configuration of a node equipment in a conventional ring type ATM (Asynchronous Transfer Mode) shown in "Local ATM Communication System (MARS [Magnetic Automatic Reservation Ssytem] network)", IEICE Technical Report SSE92-187, The Institute of Electronics, Information and Communication Engineers. In the drawing, reference numeral 1 means the node equipment, 2 is a cell switch portion to exchange a cell, 3 is a ring interface portion connected to another node equipment 1 by using a dual ring as a medium, 4 is a port interface portion employing an ATM cell transmission procedure, connected to ATM terminals, 5 is a bus to link portions forming the node equipment 1, and 6 is a manage/control portion to manage and control the respective portions through the bus. Further, reference numeral 7 means an ATM interface employing a procedure for communication with the ATM terminal to transmit and receive the cell, 8 is a cell switch to decide and output a path for the inputted cell, 9 is an SDH terminator to execute an SDH (Synchronous Digital Hierarchy) transmission procedure between one node equipment 1 and the other node equipment 1, 10 is a TC processing to control a cell header and control loop back for the dual ring, and 11 is a ring access to control cell access to the ring. The node equipment means the equipment which can switch ATM cells.
FIG. 36 is a diagram of a configuration of a network system using the node equipment 1, specially showing a state in which no abnormality is caused in a ring and the node equipment 1. Reference numeral 12 means a ring connected between the node equipment 1, and the arrow denotes a direction of signal transmission. Reference numeral 12a means a primary ring, 12b is a secondary ring, and 13 is ATM terminals employing an ATM cell transmission procedure, connected to the node equipment 1. FIG. 37 is a diagram of a configuration of the same network system as that in FIG. 36, specially showing a state in which abnormality is caused in a transmission line. The mark X means occurrence of a malfunction, and the solid line is a transmission line used for cell transmission. In the illustrative configuration, the ring includes five node equipments 1a to 1e. In the following discussion, the respective node equipments 1 are referred to as node A, node B, node C, node D, and node E.
FIG. 38 is a diagram showing a cell format serving as unit of information transmission. In the drawing, reference numeral 14 means a cell including 53 bytes, 15 is a header containing cell control information, 16 is an information field including 48 bytes and serving as an area for information transmission, 17 is a VPI (Virtual Path Identifier) which is an identifier of a virtual path, and serves as one of the cell control information, and 18 is a VCI (Virtual Channel Identifier) which is an identifier of a virtual connection, and serves as one of the cell control information.
A description will now be given of the operation. A method of transmitting the cell 14 will be described by way of, as an example, a case where the cell 14 is transmitted from an ATM terminal 13f to an ATM terminal 13a. The cell 14 is transmitted from the ATM terminal 13f, and the ATM interface 7 in the node C receives the cell 14 to input the cell 14 into the cell switch 8. Path information is previously set in the cell switch 8 so that a decision of path is made depending upon the path information, the VPI 17 and the VCI 18 in the header 15. If the result of decision requests transmission to another node equipment 1, the cell 14 is transmitted to the ring 12 via the ring access 11 and the ring interface portion 3. The cell 14 is transmitted in a direction of the solid line in FIG. 36, and is received by the node A connected to the destination ATM terminal 13a via the node D and the node E. In the node A, the cell 14 is transmitted via the ring interface portion 3 and the ring access 11, and is inputted into the cell switch 8 for the decision of path. After the decision of path is made in the cell switch 8, the cell 14 is outputted to the ATM interface 7 connected to the destination ATM terminal 13a. The ATM interface 7 transmits the cell 14 to the ATM terminal 13a.
The conventional network includes the dual ring 12 in order to enhance reliability. The secondary ring 12b in the two rings 12 serves as a stand-by ring. When the ring 12 and the node equipments 1 are normal, the cell 14 is transmitted by using only the primary ring 12a as shown by the solid line in FIG. 36. That is, the cell 14 is transmitted from the node A to the node C along a path in the order of 1a, 1b, and 1c. As shown in FIG. 37, a malfunction may be caused between, for example, the node B and the node C in the ring 12. In this case, the cell 14 is looped at the node equipments 1b and 1c disposed at both ends of the malfunction. That is, the cell 14 is looped at the node equipment 1b from primary ring 12a to the secondary ring 12b, and is looped at the node equipment 1c from the secondary ring 12b to the primary ring 12a, thereby forming one ring as shown by the solid line. This is called a loop back mode. The loop back is carried out in the TC processing 10. In a loop back state shown in FIG. 37, the cell 14 is transmitted from the node A to the node C along a path in the order of 1a, 1b, 1a, 1e, 1d, and 1c.
The conventional ring type ATM network system is provided as set forth above. Hence, there are problems in that the number of passing node equipment is more increased in the loop back state than would be in a normal state, a distance of the transmission path is more extended, and a longer transmission delay is caused between the node equipments. In some combinations of the node equipments, the transmission delay may be increased up to approximately twice a transmission delay in the normal state. Further, even in the normal state, since the one ring of the dual ring is exclusively used, the cell should be transmitted along a path having a long transmission delay in some combinations of the node equipments.